Remove subagent instructions from system prompt

[tlongwell-block]

The subagent guidance in the system prompt is redundant with comprehensive instructions embedded directly in the tool descriptions themselves. The dynamic_task__create_task tool already explains the isolation context, extension handling, and return behavior, while subagent__execute_task details the execution strategies with clear examples.

Rather than maintaining duplicate instructions that could drift out of sync, consolidating this information into the tool descriptions provides a cleaner, more maintainable approach that reduces cognitive overhead while preserving all necessary guidance for effective subagent usage.